Difficult to tell, if you wet the underside and take a picture it should be easier to see. It sounds silly but it will make it stand out more if its shell, ammonite or just the mineral calcite. Same sort of picture as the forth pic only wet.

If you are able to bring or post the specimen to the AMC centre at the museum our Earth Science officer would be more than happy to look at it for you to see if they can help solve the stone or fossil mystery.